U.S. patent number 6,352,349 [Application Number 09/535,051] was granted by the patent office on 2002-03-05 for illumination system for use in imaging moving articles.
This patent grant is currently assigned to United Parcel Services of America, Inc.. Invention is credited to Mark B. Braginsky, Robert H. Esslinger, Peter R. Gluege, William D. Hess.
United States Patent |
6,352,349 |
Braginsky , et al. |
March 5, 2002 |
**Please see images for:
( Certificate of Correction ) ** |
Illumination system for use in imaging moving articles
Abstract
An apparatus and method for illuminating articles, such as
parcels, moving beneath a camera on a conveyor belt. Multiple light
sources are positioned alongside the conveyor to direct light
angled downwardly and from the side of the conveyor. Each light
source has an elongated lamp angled with respect to the surface of
the article and a light directing device, preferably a Fresnel
lens, positioned to receive light from the lamp and to direct the
light toward the surface at an angle equal to or greater than forty
degrees from an optical axis of the camera.
Inventors: |
Braginsky; Mark B. (Longmeadow,
MA), Esslinger; Robert H. (Wilton, CT), Hess; William
D. (Clinton Corners, NY), Gluege; Peter R. (Warwick,
NY) |
Assignee: |
United Parcel Services of America,
Inc. (Atlanta, GA)
|
Family
ID: |
24132639 |
Appl.
No.: |
09/535,051 |
Filed: |
March 24, 2000 |
Current U.S.
Class: |
362/8; 362/11;
362/16; 362/18 |
Current CPC
Class: |
G06K
7/10732 (20130101) |
Current International
Class: |
G06K
7/10 (20060101); G03B 015/02 () |
Field of
Search: |
;362/8,11,16,17,18,146 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
UV Filters/Light Control Film, Mounted And Unmounted Glass UV
Filters, Industrial Optics Division; pp. 65 and 66. .
Allard Graphic Arts, Fresnel Lens, pp. 1068-1070..
|
Primary Examiner: O'Shea; Sandra
Assistant Examiner: Delgizzi; Ronald E.
Attorney, Agent or Firm: Alston & Bird LLP
Claims
What is claimed is:
1. In a system for conveying articles on a conveyor in a direction
of travel under a camera, an apparatus for illuminating an upper
surface of said articles, comprising:
an elongated light source positioned alongside said conveyor, with
a longitudinal axis of said light source forming an angle from the
horizontal;
said light source including a lamp and an elongated reflector
positioned to reflect light from said light source toward articles
on said conveyor; and
a light directing device positioned between said light source and
said conveyor to receive light from said light source and to direct
the light toward articles on said conveyor at an angle equal to or
greater than forty degrees from an optical axis of said camera;
and
a barrier positioned to confine the light illuminating said
articles to light passing through said light directing device.
2. The system of claim 1, wherein the longitudinal axis of said
light source is approximately perpendicular to said path of
travel.
3. The system of claim 1, wherein said reflector is an elliptical
surface and wherein said lamp is an elongated lamp lying along a
focal axis of said elongated reflector.
4. The system of claim 3, wherein a second focal axis of said
elongated reflector lies a distance beneath said conveyor.
5. The system of claim 1, wherein said light directing device
comprises a Fresnel lens.
6. The system of claim 5, wherein said Fresnel lens is positioned a
distance from said lamp from about 1/4 to about 1/2 times the focal
length of said Fresnel lens.
7. The system of claim 6, wherein the optical axis of said Fresnel
lens is offset from the axis of symmetry of said reflector.
8. The system of claim 7, wherein the optical axis of said Fresnel
lens is offset from the axis of symmetry of said reflector by a
distance from about 0.5 to 1 times the length of said lamp.
9. The system of claim 8, wherein said Fresnel lens is positioned a
distance from a central axis of said conveyor approximately equal
to 1 to 2 times the focal length of said Fresnel lens.
10. The apparatus of claim 7, wherein said Fresnel lens is offset
from the axis of symmetry of said reflector to a position farther
from said conveyor.
11. The system of claim 10, wherein said assembled light source and
Fresnel lens comprise a first illumination assembly and further
comprising a second illumination assembly positioned alongside said
conveyor above said first illumination assembly.
12. The system of claim 11 further comprising a third and a fourth
illumination assembly positioned above one another, across said
conveyor from said first and second illumination assemblies.
13. The system of claim 1, wherein said assembled light source and
Fresnel lens comprise a first illumination assembly and further
comprising a second illumination assembly positioned alongside said
conveyor above said first illumination assembly.
14. The system of claim 13, further comprising a third and a fourth
illumination assembly positioned above one another, across said
conveyor from said first and second illumination assemblies.
15. The system of claim 1, wherein said assembled light source and
Fresnel lens comprises a first illumination assembly and further
comprising a second illumination assembly positioned across said
conveyor from said first illumination assembly.
16. The system of claim 1, wherein said lamp, reflector, light
directing device, and barrier are selected, assembled and oriented
such that said articles are illuminated from the side of said
conveyor, and such that essentially all of the light exiting said
light directing device is aligned at an angle equal to or greater
than forty-five degrees from the optical axis of said camera, and
such that diffuse reflection but essentially no specular reflection
from said articles of light from said light source reaches said
camera.
17. The system of claim 1, wherein said light directed by said
light directing device forms an illuminated strip on an upper
surface of said articles transverse to said direction of
travel.
18. The system of claim 1, where said light directing device
comprises a set of multiple plates positioned on an article side of
the lamp opposite the reflector to form narrow slots across the
reflector, perpendicular to the filament axis, portions of said
plates positioned to receive light that would reflect specularly to
said camera being diffusing surfaces.
19. An apparatus for illuminating a surface of an article being
imaged by a camera, comprising:
an elongated light source positioned alongside said article to
direct light from said light source toward said article, a
longitudinal axis of said light source forming an angle with said
surface; and
a Fresnel lens positioned between said light source and said
article to receive light from said light source and to direct the
light toward said article at an angle equal to or greater than
forty degrees from an optical axis of said camera;
said light source being located closer to said Fresnel lens than
the focal length of said Fresnel lens, and the optical axis of said
Fresnel lens being offset from an axis of symmetry of said light
source.
20. The apparatus of claim 19, wherein said light source includes
an elongated lamp and an elongated reflector, and wherein said
Fresnel lens is positioned a distance from said lamp from about 1/4
to about 1/2 times the focal length of said Fresnel lens, and the
optical axis of said Fresnel lens is offset from the axis of
symmetry of said reflector by a distance from about 0.5 to 1 times
the length of said lamp.
21. The apparatus of claim 20, wherein said Fresnel lens is offset
from the axis of symmetry of said reflector to a position farther
from said article.
22. A method for illuminating articles moving on a conveyor in a
direction of travel under a camera, comprising the steps of:
directing a beam of light from the side of said conveyor at a
downward angle toward said articles; and
collimating said beam to direct the light at an angle equal to or
greater than forty-five degrees from an optical axis of said camera
and to form an illuminated strip on an upper surface of said
articles transverse to said direction of travel.
23. The method of claim 22 wherein said step of collimating said
beam comprises collimating said beam with a Fresnel lens.
24. In a system for conveying articles on a conveyor in a direction
of travel under a camera, an apparatus for illuminating an upper
surface of said articles, comprising:
an elongated light source positioned alongside said conveyor, with
a longitudinal axis of said light source forming an angle from the
horizontal;
said light source including a lamp and an elongated reflector
positioned to reflect light from said light source toward articles
on said conveyor; and
said reflector being part of a cone-shaped figure having a first
focal axis approximately colinear with said lamp, and a second
approximately horizontal focal axis adjacent to said conveyor.
Description
TECHNICAL FIELD
The present invention relates to camera lighting systems, and more
particularly relates to an apparatus and method for illuminating
articles moving beneath a camera on a conveyor belt.
BACKGROUND ART
Automatic conveyor systems have been developed for handling and
sorting articles, such as parcels or components of manufactured
goods. The articles bear identifying indicia, for example, bar
codes, dense two-dimensional symbols, and text suitable for optical
character recognition. An overhead linear CCD (Charge Coupled
Device) camera can capture narrow images of a field of view across
a "scan line" that is very short in the direction of conveyor
travel, but extends across the conveyor. A computer receiving the
output of the camera can build from these linear images a full
digital image of a parcel or a label passing under the camera. When
the articles have varying heights, the camera can take an image
anywhere in a "scan plane" projected downward from the camera to
the scan line.
Prior illumination systems have produced an intensive strip of
light along the scan line by using elliptical cylinders as
reflectors behind an elongated tubular lamp. The lamp lies along
one focal axis of the ellipse, while the surface to be illuminated
lies near the other focal axis. The light source may be inclined
about the axis of the scan line so as not to obstruct the camera's
field of view. Generally, the region of focused bright illumination
is not vertically deep within the scan plane, and thus the efficacy
of this configuration is limited when the articles passing beneath
the camera vary widely in height.
Such an illumination system seeks to direct enough light reflected
from the subject article to the camera to obtain a sharp image,
without blinding the camera with glare. Thus, the light sources
should provide a large amount of diffuse reflection to the camera,
but no specular reflection. However, articles with shiny upper
surfaces present a particular challenge. If the angle of
inclination of the light source about the scan line is small, the
camera may be blinded by glare reflected from glossy packages or
from plastic protectors used to protect paper labels. If the angle
of inclination is large, tall articles may cast shadows onto
leading or trailing articles when they are under the camera. At an
optimum angle close to forty-five degrees, the light source can
cover only a relatively small depth vertically in the scan plane,
because elliptical reflectors cast a narrow beam transverse to the
length of the reflector. One prior system provides an unsymmetrical
elliptical reflector to illuminate a vertical region in the scan
plane, but this approach does not solve all the problems noted
above.
Patents disclosing illumination systems include U.S. Pat. Nos.:
3,569,961 3,809,462 3,982,116 4,689,490 4,733,335 5,022,740
5,040,883 5,245,411 5,308,960 5,313,373 5,388,035 5,430,282
5,521,365 5,538,065 5,600,116 5,607,229 5,777,743 5,791,771
5,818,528 5,821,518
There is a need in the art for an illumination system capable of
providing adequate diffuse light to an overhead camera, reflected
from a vertical region sufficient to accommodate a range of article
heights. The illumination system should avoid reflecting glare to
the camera, and should direct light so that tall articles do not
cast shadows into the illuminated region.
SUMMARY OF THE INVENTION
The present invention seeks to provide a system and method for
illuminating a surface of an article being imaged by a camera so as
to reflect diffuse light to the camera from articles of varying
heights.
In accordance with the invention, this object is accomplished in an
illumination system by providing an apparatus for illuminating a
surface of an article being imaged by a camera, comprising an
elongated light source angled with respect to the surface of the
article and positioned alongside the article to direct light toward
the surface; and a light directing device positioned to receive
light from the light source and to direct the light toward the
surface at an angle equal to or greater than forty degrees from an
optical axis of the camera. By constraining essentially all of the
light rays to angles 40 degrees or more from the optical axis of
the camera, the system avoids a level of specular reflection or
glare that would deteriorate the image of symbols or text on the
surface of the articles to the extent of preventing reliable
decoding or reading of such symbols or text. When the invention is
incorporated in a conveyor system in which the articles are moved
on a conveyor, the light source is positioned alongside the
conveyor to direct light angled downwardly and from the side of the
conveyor. The light directed by the light directing device forms an
illuminated strip on an upper surface of the articles transverse to
their direction of travel.
In one embodiment of the invention, the light source is an
elongated lamp and an elongated reflector positioned to reflect
light from the lamp toward the articles, and the light directing
device is a Fresnel lens. Preferably, the Fresnel lens has an
optical axis that is offset from an axis of symmetry of the light
source. To avoid specular reflection into the camera, the Fresnel
lens bends light from the light source to a larger angle with
respect to the optical axis of the camera. The optical axis of the
lens is offset from the axis of symmetry of the light source away
from the article along the direction of the optical axis of the
camera to more efficiently illuminate both short and tall articles.
A barrier, such as a housing surrounding the light directing
device, may be used to block light from the light source except
light passing through the light directing device.
The invention also provides an automatic conveyor system in which
the longitudinal axis of the lamp is approximately perpendicular to
the path of travel of articles along the conveyor. The reflector
preferably is an elliptical surface and the lamp lies along a first
focal axis of the elongated reflector. The second focal axis of the
reflector preferably lies a distance beneath the conveyor. In the
preferred embodiment, the Fresnel lens is positioned a distance
from the lamp from about 1/4 to about 1/2 times the focal length of
the Fresnel lens and a distance from a central axis of the conveyor
approximately equal to 1 to 2 times the focal length of the Fresnel
lens, and the optical axis of the Fresnel lens is offset from the
axis of symmetry of the reflector by a distance from about 0.5 to 1
times the length of the lamp filament.
In a preferred embodiment, the illumination system includes a
plurality of illumination assemblies, including light sources and
light directing devices, positioned to provide light to the
illuminated region of the camera's scan plane. Optimally, four such
assemblies are provided, the first and second assemblies on one
side of the conveyor, one above the other, and the third and fourth
assemblies on the opposite side of the conveyor, one above the
other. When only one illumination assembly is present, preferably
its symmetry plane substantially coincides with the camera's scan
plane. When a plurality of illumination assemblies are used, they
may be aligned with the camera's scan plane to maximize
illumination. Optionally, they may be positioned with their
symmetry planes slightly mis-aligned to create a thickness in the
illuminated region spanning the scan plane, and thus in the
illuminated strip on the surface of the article.
In preferred embodiments of the invention, the lamps, reflectors,
lenses, and barriers are selected, assembled and oriented such that
the articles are illuminated from the side of the conveyor, such
that essentially all of the light exiting the lens is aligned at an
angle equal to or greater than forty-five degrees from the optical
axis of the camera, and such that diffuse reflection but
essentially no specular reflection from the articles reaches the
camera. Because the light sources are to the side of the conveyor,
tall articles cannot cast shadows on leading or trailing articles.
The preferred arrangement of multiple light sources illuminates a
deep region of the scan plane without the need to position any
light source so high as to create glare into the camera.
The present invention also provides a method for illuminating
articles moving on a conveyor in a direction of travel under a
camera, comprising the steps of directing a beam of light from the
side of the conveyor at a downward angle toward the articles; and
collimating the beam to direct the light at an angle equal to or
greater than forty degrees from an optical axis of the camera and
to form an illuminated strip on an upper surface of the articles
transverse to the direction of travel.
In an alternative embodiment, a pair of additional illumination
assemblies may be installed within the footprint of the camera's
field of view, if necessary for tall articles. In another
alternative configuration for any of the above embodiments, the
reflector of the light source may be part of a cone-shaped figure,
rather than part of a cylinder. As a result, the lamp is inclined
with respect to the second focal axis of the reflector. This
configuration allows the light to be focused parallel to the scan
line despite the inclination of the lamp.
In another alternative embodiment, the light directing device can
be a set of multiple apertures forming narrow slots across the
reflector, under the lamp and perpendicular to the filament axis.
Reflective and diffusing surfaces within the slots are configured
to avert direct illumination of the articles by beams having an
angle of incidence less than forty degrees from the camera's
optical axis, and to create useful diffuse light from undesirable
direct light beams.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a conveyor system including an
illumination apparatus embodying the present invention.
FIG. 2A is an exploded pictorial view of an illumination assembly
including cooling systems for the lens and for the lamp
FIG. 2B is a cross sectional view taken along line 2B--2B of FIG.
2A.
FIG. 3 is a diagrammatic exploded view of the components of the
illumination assembly and their relation to the camera.
FIG. 4 is diagrammatic top view of the light source and its
relation to an article being imaged.
FIG. 5 is a diagrammatic partial front view of a conveyor system
with two illumination assemblies shown on one side of the
conveyor.
FIG. 6 is a front view of a Fresnel lens for use with the present
invention.
FIG. 7 is a diagrammatic, enlarged, partial side view of an
illumination assembly showing the refractive function of the
Fresnel lens.
FIG. 8 is a diagrammatic front view of another embodiment of the
invention incorporating an alternative light directing device.
FIG. 9 is a detailed diagrammatic view of the light directing
device and light source of FIG. 8.
FIG. 10 is a diagrammatic view of an alternative reflector for use
with vertical lamp filaments.
FIG. 11 is a diagrammatic view of an alternative cone-shaped
reflector configured to compensate for a tilted lamp.
FIG. 12 is a perspective view showing positioning of additional
illumination assemblies in another embodiment of the invention.
FIG. 13 is a side view of the embodiment of FIG. 12.
DETAILED DESCRIPTION
Referring now in more detail to the drawings, in which like
numerals refer to like parts throughout the several views, FIG. 1
shows a conveyor system 10 including an illumination apparatus 11
embodying the present invention. The conveyor system 10 includes a
conveyor belt 12 that travels in the direction of the arrow T on a
conveyor bed 14, and an imaging station 15 that includes the
illumination apparatus 11. In the imaging station, a superstructure
17 straddles the conveyor belt 12 and supports a camera 18 having
an optical axis D-D directed vertically down to the conveyor
belt.
The camera 18 preferably is a linear CCD camera. The field of view
of the camera has a width encompassing the width of the conveyor
belt 12, and a length or thickness in the direction of travel T,
for example 0.00535 inch (0.013589 cm). The camera scans a narrow
strip (scan line SL) across the width of the conveyor belt 12 or
across the upper surface of an article moving with the conveyor
belt. Such a camera system is described in U.S. Pat. No. 5,308,960.
The camera preferably has a long depth of focus or an automatic
focusing apparatus, such as described in U.S. Pat. No. 5,245,172 or
U.S. Pat. No. 5,485,263. As shown in FIG. 5, the articles traveling
on the conveyor may be parcels, such as parcel 100 carrying a label
110 containing optically encoded symbols (for example, bar codes or
two-dimensional dense codes such as the MaxiCode.RTM. symbol) or
machine-readable text suitable for optical character recognition
(OCR). The parcel will pass through the scan plane (projected
vertically above the scan line) at a height determined by the
height of the parcel. The illumination apparatus 11 must adequately
illuminate the label 110 in the region SL spanning the scan line
(shown in dotted outline on the conveyor in FIG. 1) regardless of
whether the parcel is a flat envelope or the tallest parcel
acceptable in the conveyor system 10. The term "article" used
herein means an object or a label attached to an object.
In a known manner, the camera 18 acquires linear images of small
strips within its field of view and assembles them in a computer
memory to form a two-dimensional digital image of the top of the
parcel 100 including the label 110. Computer software also known in
the art then analyzes the image to locate and decode symbols and
text in the image to assist in recognition and sorting of the
parcel 100. The conveyor system 10 can also be used for processing
components of manufactured goods or other articles carrying
identifying indicia or having a shape discernable in an image.
The illumination apparatus 11 includes at least one lighting module
20. In the system shown in FIG. 1, four lighting modules 20a-20d
illuminate the region SL. An upper left module 20a and a lower left
module 20b are mounted in a left hood 22a attached to the
superstructure 17 on the left side of the conveyor belt 12, as
viewed in FIG. 1. An upper right module 20c and a lower right
module 20d are mounted in a right hood 22b attached to the
superstructure 17 on the right side of the conveyor belt 12. Each
lighting module 20 is contained in a housing 27 that is secured at
no less than about a forty degree angle with respect to the
camera's optical axis D-D, and preferably at about a forty-five
degree angle. The lighting modules and hoods are mounted alongside
the conveyor, that is, substantially outside the footprint of the
camera's field of view.
As shown in more detail in FIG. 2, the housing 27 defines a light
chamber 29, a lens chamber 30 between the light chamber 29 and the
conveyor belt 12, and a heat exchange chamber 32 adjacent to the
lens chamber. At the boundary between the lens chamber 30 and the
lighting chamber 29, the lighting chamber is enclosed by a glass
plate 42. The light chamber 29 includes a plurality of spaced-apart
struts 34 each defining an elliptical edge 35 facing the lens
chamber 30. An elliptical reflector 38 fits against the edges 35
and fills the cross-section of the light chamber 29. At a first
focal axis of the elliptical reflector 38, an elongated sodium lamp
40 is mounted within the lighting chamber. The reflector preferably
is about 10-15 inches (about 25-38 cm) long, and the distance
between the elliptical focal axes is about 63 inches (about 160
cm). The length a of the lamp 40 preferably is about 5 inches
(about 12.7 cm). The reflector 38 is positioned, and its curvature
selected, so that a second focal axis of the reflector preferably
is about one inch (about 2.5 cm) below the conveyor belt 12. These
parameters may be varied to suit different conveyor systems and
different types or sizes of articles expected to be processed.
A Fresnel lens 45 encloses the end of the lens chamber 30 opposite
the glass plate 42. The chamber 30 preferably is empty, its walls
providing a barrier to allow light from the lamp 40 and the
reflector 38 to pass toward the conveyor 12 only through the lens
45. Acrylic plastic Fresnel lenses are commercially available
having a 35 inch (89 cm) diameter and 30 inch (76 cm) focal length
f. The lens 45 may be cut to fit the opening of the lens chamber
30, preferably about 8 inches (about 20.3 cm) wide by about 18
inches (about 45.7 cm) long. However, the lens is cut to fit the
opening with the optical axis of the lens offset from the center of
the chamber, as shown in FIG. 6. Preferably, the optical axis is
approximately centered on the width of the chamber 30, but offset
from the center along the length of the chamber a distance b from
about 0.5 to 1 times the length of the lamp 40. In the preferred
lighting assembly, the lamp 40 is about 5 inches (about 12.7 cm)
long and the optical axis of the lens 45 is offset about 4 inches
(about 10.2 cm). As best shown in FIG. 3, the depth of the lens
chamber 30 is selected so that the distance r from the lamp 40 to
the lens 45 is between about one-quarter and one-half the focal
length of the lens 45. When the focal length f is 30 inches (76
cm), the distance r preferably is about 8 inches (about 20 cm).
The refraction of the light from the lamp 40 by a portion of the
Fresnel lens 45 is shown diagrammatically in FIG. 7. An example ray
R.sub.S emanating from the lamp is shown reflecting from the
reflector 38 along a path (shown in dashed line) that would reflect
specularly from an article on the conveyor to the camera 18.
However, the triangular rib of the Fresnel lens 45 bends the ray to
a path R.sub.D that makes an angle .phi. of at least 40 degrees
with the optical axis D-D of the camera 18. Thus, the rays R.sub.D
projected by the lens 45 toward the article on the conveyor will
create diffuse reflection to the camera, not glare. Of course,
light passing directly from the lamp to the lens is refracted
similarly.
The preferred configuration of the light source and light directing
device with respect to the conveyor 12 is shown diagrammatically in
FIGS. 3 and 4. A parcel 100 is shown on the conveyor 12. The axis
of symmetry C-D of the reflector 38 makes an angle of about 45
degrees with the optical axis D-D of the camera 18. The lamp 40
lies along the first focal axis A-A of the reflector 38. The
Fresnel lens 45 is spaced a distance r in front of the lamp, with
its optical axis F-F parallel to but offset from the axis C-D by a
distance b in a direction (along the camera axis D-D) away from the
parcel 100. The lens 45 is positioned a distance s away from the
scan line SL on the conveyor belt 12, the distance s being between
about the focal length f and twice the focal length f. The second
focal axis B-B of the elliptical reflector 38 preferably falls
slightly below the conveyor belt, as shown in phantom in FIG.
4.
FIG. 5 shows, diagrammatically in cross section at the scan line
SL, the preferred configuration of the illumination apparatus 11,
looking back along the path of travel of the conveyor belt 12. The
arrows R indicate the path of rays of light from the lamps 40 and
the reflectors 38 of the lighting modules 20c and 20d on one side
of the conveyor, the other side of the conveyor being substantially
a mirror image. As shown, all the illuminating rays make an angle
.phi. of 40 degrees or more, and preferably 45 degrees or more,
with the optical axis D-D of the camera 18. This configuration
provides adequate diffuse illumination to the camera for a sharp
image, while avoiding specular reflection.
Returning to FIG. 2, the housing 27 contains two cooling systems,
one for the lamp 40, and the other for the lens 45, which would
otherwise melt in a short time. In the light chamber 29, a pair of
heat exchange tubes 47 extend through the struts 34 on either side
of the convex surface of the reflector 38. The tubes 47, made of a
high heat conductivity material such as copper, are open to the
outside air at one end of the chamber 29, and terminate at a thin
plenum 48 at the other end of the chamber. The plenum 48 has a pair
of openings 50 that communicate only with the open ends of the
tubes 47. A fan 49 pressurizes the plenum 48 to force cooling air
through the tubes. Heat generated by the lamp 40 travels through
the reflector into the light chamber 29 surrounding the tubes, and
then through the walls of the tubes into the cooler air passing
through the tubes.
A recirculating air system cools the lens 45. Another fan 51 is
mounted in the heat exchange chamber 32 adjacent to an inlet
opening 52 connecting the chamber 32 to the lens chamber 30. The
fan 51 is mounted at an angle to direct air downwardly onto the
lens 45. The air sweeps around the chamber 30, cooling the lens 45,
and is exhausted by a similarly tilted fan 54 through an outlet
opening 55 at the opposite end of the chamber 30. A plurality of
heat exchange tubes 57 extend across the chamber 32 at its end
opposite the lens chamber 30. The tubes 57, made of a high heat
conductivity material such as copper, are open to the outside air
at one end of the chamber 32, and terminate at a thin plenum 59 at
the other end of the chamber. A fan 60 pressurizes the plenum 59 to
force cooling air through the tubes 57. The air exhausted from the
lens chamber 30 by the fan 54 sweeps around the chamber 32 and over
the heat exchange tubes 57, transferring heat to the cooler air
flowing within the tubes. The fan 51 then forces the cooler air
back across the lens 45. If desired, appropriate baffles can be
positioned in the lens and heat exchange chambers to direct the
flow of recirculating air.
The recirculating air cooling systems with heat exchangers are
enclosed to prevent dust from contacting the optics.
In operation, the illumination apparatus 11 projects light from the
sides of the conveyor belt 12 as a parcel 100 or other article
travels under the imaging station 15. With the lamp 40, reflector
38, and lens 45 configured within the limits described above, light
shining on the parcel is angled at 40 degrees or more from the
camera's optical axis. As shown in FIGS. 1 and 5, the projected
light illuminates a region across the conveyor including the scan
line SL of the camera 18 and extending above the conveyor as high
as the highest expected parcel. As the parcel passes through the
illuminated region, diffuse light reflected from the parcel, and
particularly from a label 110 bearing optically encoded symbols and
sometimes machine-readable text, reaches the camera 18. Specular
reflection is directed away from the camera, avoiding glare. The
camera acquires a series of narrow images, which its processor
builds into a digital image of the entire field of view. Software
known in the art can analyze the image to find symbols and text
blocks, and then to decode the information on the label. During
operation of the lamp 40, the cooling systems shown in FIG. 2
protect the lens from overheating.
FIG. 8 shows a second embodiment of an illumination apparatus 211
according to the present invention, incorporating an alternative
light directing device. Two different height parcels 100 are shown
on the conveyor belt 12. They are illuminated by two lighting
modules 220a and 220b, which are similar to the modules 20 of the
first embodiment, except that the Fresnel lens is replaced by a set
of baffle plates 245, shown in detail in FIG. 9. A plurality of
parallel baffle plates 248 are positioned perpendicular to the lamp
40 and parallel to the axis of symmetry of the reflector 38.
Preferably, each module has twelve baffle plates extending about 8
inches (20 cm) parallel to the axis of symmetry of the reflector,
and positioned about 0.75 inch (1.9 cm) apart, to form a series of
slots for receiving light directly from the lamp and also reflected
from the reflector. The plate surface on one side of each slot is a
completely specular or mirror-like surface 252. In the
configuration shown in FIG. 8, the fully specular surface 252 of
each plate 248 faces slanted upwardly with respect to the camera's
optical axis D-D, as a result of the tilting of the light source.
The other, downwardly facing surface is divided into two parts on
either side of a midpoint 250, a specular portion 253 nearest to
the lamp 40, and a diffuse portion 254. The portion 254 may be made
irregular in a manner known in the art so that light reflects
diffusely from the surface.
In FIG. 9, the first two slots are shown being traversed by beams
R.sub.a leaving the lamp and the reflector at angles of +5 and -5
degrees. These beams pass through the slot without touching the
plates, and will reach a target parcel or other article at angles
of 40 and 50 degrees, respectively, as a result of the tilting of
the light source. The next two slots show beams R.sub.b leaving the
lamp and the reflector at angles of +10 and -10 degrees. These
beams hit the plates 248 defining the slot near their outer edges.
The lower specular surface 252 reflects a 10 degree beam and
redirects it toward the parcel at a 55 degree angle. The upper
diffuse surface 254 spreads a 10 degree beam diffusely on the
parcel. The next two slots show beams R.sub.C leaving the lamp and
the reflector at angles of +20 and -20 degrees. These beams will
undergo at least two reflections within a slot. Any beam that would
undergo a specular reflection aiming it at the parcel at an
undesirable angle (greater than 40 degrees) will undergo a diffuse
reflection before exiting the slot. Beams of light that pass above
the plate set 245 are at angles .theta. equal to or greater than 40
degrees to the camera optical axis.
While the embodiment of FIGS. 8 and 9 is effective, the first
embodiment using a Fresnel lens is more efficient in use of light
energy output by the lamps.
Some high intensity lamps useful for illuminating articles work
only in a horizontal or vertical position, so the lamp itself
cannot be tilted. FIG. 10 shows a lighting module 320 including a
lamp 40 in a vertical configuration flanked by a set of plates 248
angled at 45 degrees toward the target article. An elliptical
reflector 338 defines rib-like segments having an angle of
inclination of 22.5 degrees to the lamp axis. Light from the lamp
that strikes these angle segments is reflected into the slots
formed by the plates 248. Thus, the light from the vertical lamp is
directed into the light directing device, which operates in the
manner described above in connection with FIGS. 8 and 9. It will be
apparent that the same approach can be used to form a ribbed
reflector that can direct light from a horizontal lamp into the
light directing device, and that these embodiments can be used on
both sides of a conveyor belt.
An alternative shape for any of the reflectors described above is
shown in FIG. 11. In this embodiment, the elliptical reflector 438
is part of a cone-shaped FIG. 401, rather than part of a cylinder
as is the reflector 38 shown in FIG. 4. As a result, the lamp 40
lying along the first focal axis F1-F3 is inclined with respect to
the second focal axis F2-F4. This configuration allows the light to
be focused parallel to the scan line SL despite the inclination of
the lamp.
In some circumstances it may be necessary to originate illumination
from within the footprint of the camera's field of view to
illuminate tall articles. In such a case the preferred
configuration is shown in FIG. 12 and FIG. 13, looking from the
side of the conveyor 12. Two additional lighting modules 20e and
20f are installed, one on each side of the centerline of the
conveyor, but inside the lateral position of the modules 20a and
20c. The reflectors 538 and 539 of the modules 20e and 20f are
angled laterally outwardly at about 45 degrees from the camera
optical axis, like the other reflectors 38. However, the reflectors
538 and 539 also are angled by relatively small angles .alpha. and
.gamma. out of the scan plane. Preferably, the angle .alpha. is
about 15 degrees behind the scan plane, and the angle .gamma. is
about 20 degrees ahead of the scan plane. The reflector 538 is
focused at secondary axis F538, farther below the conveyor belt 12
than the secondary axis F539 or the reflector 539. Thus, each
reflector 538, 539 illuminates a certain area of height span above
the scan line.
From the foregoing description, it can be seen that the present
invention provides illumination to an overhead imaging camera by
reflecting diffuse light to the camera from articles of varying
heights. The main embodiments of the invention have the advantage
of originating the light rays from outside the footprint of the
field of view of the camera. By constraining essentially all of the
light rays to angles 40 degrees or more from the optical axis of
the camera, the system avoids a level of specular reflection or
glare that would deteriorate the image of symbols or text on the
surface of the articles. Furthermore, the invention makes novel use
of Fresnel lenses to collimate and control the light rays,
providing diffuse illumination with minimum loss of light
energy.
While this invention has been described in detail with particular
reference to preferred embodiments thereof, it will be understood
that modifications and variations may be made without departing
from the scope of the invention as defined in the appended
claims.
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